This application is a continuation of Ser. No 817,614, filed Jan. 10, 1986, abandoned. 1. Field of the Invention
The present invention relates to scintillation cameras for medical diagnoses of subjects and the like by the use of radioactive rays and, more particularly, relates to scintillation cameras in which the change in sensitivity of a photomultiplier tube of a radiation detector is continuously detected and automatic adjustment is performed so as to prevent the sensitivity of the photomultiplier tube from fluctuating.
2. Description of the Prior Art
Generally, a scintillation camera is an apparatus arranged such that distribution of a radioisotope or the like injected into a subject, e.g., a human body, is detected by a radiation detector, the electric signal from the detector is transformed into coordinates (X, Y) by a position calculating circuit, and a distribution image of incident radioactive rays is displayed on a display unit such as a cathode-ray tube or the like. Recently, uniformity of an image in scintillation cameras of this kind has been improved by the introduction of correcting means utilizing micro-computers. Since the correcting means of this type is used to improve the uniformity of the image by correcting image signals of the scintillation camera with correction data made by measuring uniformity of the scintillation camera body per se, the correction data often becomes unsuitable owing to variations in uniformity of the scintillation camera body per se, resulting in deterioration in uniformity of the image.
A main cause of the change in uniformity of the scintillation camera body is variations in sensitivity of photomultiplier tubes in the radiation detector with the passage of time. To cope with this problem, conventionally, removal of fluctuation in the sensitivity of the photomultiplier tubes has been attempted in the following manner. That is, as shown in FIG. 5, in a scintillation camera including a scintillator 1, a plurality of photomultiplier tubes 2, a position calculating circuit 3, and a display unit 4, there is provided a circuit for the detection adjustment of variations in sensitivity of the photmultiplier tubes 2, the circuit being constituted by a changeover switch 5, a signal changeover circuit 6, an averaging circuit 7, a correction value calculating circuit 8, a correction value memory 9, a correcting circuit 10, and a lamp 11. The adjusting operation of the circuit will be described in detail hereunder.
First, a radiation source such as a radioisotope or the like is packed in a cylindrical case 12 made of lead and having one end perforated for emitting fine beam radiation of rays such as .gamma.-rays or the like, and this lead case 12 is disposed just under a desired one of the photomultiplier tubes 2 the sensitivity of which is to be adjusted. Next, the signal changeover circuit 6 is changed over by the changeover switch 5 such that only the output signal from the above-mentioned desired one photomultiplier tube 2 is supplied to the averaging circuit 7, among the outputs of the plurality of photomultiplier tubes 2. This averaging circuit 7 calculates an arithmetical average of the respective magnitudes of the output signals of the desired one photomultiplier tube 2 with respect to a predetermined number of incident radioactive rays (e.g., 1000 of rays), so as to remove the statistical variations of the output signal of the photomultiplier tube 2. Next, the arithmetical average from the averaging circuit 7 is supplied to the correction value calculating circuit 8. This correction value calculating circuit 8 calculates the difference between the arithmetical average and a reference value datermined in accordance with the energy of incident radioactive rays, the difference corresponding to the variations in sensitivity of the photomultiplier tube 2 to be adjusted. Thus, the lead case 12 with the radiation source is disposed just under every one of the photomultiplier tubes one after another and every time the lead case 12 is displaced, the signal changeover circuit 6 is also changed over by the changeover switch 5 to take in the output signals from the photomultiplier tube 2 to be adjusted. In this manner, all the photomultiplier tubes 2 are adjusted through the above-mentioned adjusting operation.
However, scintillation cameras of this kind have a disadvantage in that the sensitivity adjusting operation is complicated, since the lead case 12 with the radiation source must be moved so as to be disposed just under each one of the plurality of photomultiplier tubes 2 one after another and every time the lead case 12 is displaced the changeover switch 5 must be operated to change over the signal changeover circuit 6. In the instance where the displacement of the lead case 12 and the operation of the changeover switch 5 are automatically performed, on the other hand, there is another disadvantage in that the structure for the adjustment of sensitivity becomes complicated and large. Moreover, the sensitivity of each of the photomultiplier tubes 2 must be adjusted at a specific time other than the test time for a subject and it is thus impossible to continuously detect and remove the variations in sensitivity of the photomultiplier tube with the passage of time.